1. Field of the Invention
The present invention relates to a lithium-ion cell (in the following, the lithium-ion cell will be called as an Li-ion cell) and a lithium-ion battery (in the following, the lithium-ion battery will be called as an Li-ion battery). More specifically, the present invention relates to a capacity estimation method, a degradation estimation method and a degradation estimation apparatus for the Li-ion cell and the Li-ion battery.
2. Description of the Related Art
Recently, demand for cells is growing as various electronic devices become smaller, sophisticated and portable. According to this growing demand, cells are intensively developed and improved. Accordingly, the scope of application of cells has been extended.
As the cells are becoming widespread, demand for improvement of reliability of the cells is growing. Especially, ensuring reliability of a nickel-metal hydride cell (in the following, the nickel-metal hydride cell will be called as an Ni/MH cell) and a Li-ion cell is an important problem since energy which is accumulated in the Ni/MH cell or the Li-ion cell is higher than that of a conventional lead-acid cell or a nickel-cadmium cell (in the following, the nickel-cadmium cell will be called as an Ni/Cd cell) so that damage which may be caused by a trouble of the Ni/MH cell or the Li-ion cell is more serious than that of the lead-acid cell or the Ni/Cd cell. That is, since volumetric or gravimetric energy density of the Ni/MH cell or the Li-ion cell is much higher than that of the lead-acid cell or the Ni/Cd cell, energy which is accumulated in the Ni/MH cell or the Li-ion cell is higher than the lead-acid cell or the nickel-cadmium cell.
In addition, the Li-ion cell does not have gas absorbing reaction mechanism which absorbs gas which is generated due to side reaction of overcharging although the lead-acid cell, the Ni/Cd cell, and the Ni/MH cell have such a gas absorbing reaction mechanism. The latter cells include aqueous solution, sulfuric acid or alkaline solution, as electrolyte. Oxygen gas evolves on the cathode and the gas is absorbed in anode active materials for these cells. On the other hand, the Li-ion cell includes lithiated transition metal oxide as cathode active material which intercalates and deintercalates lithium, carbon compound as anode active material which intercalates and deintercalates lithium, and nonaqueous organic mixed solvent in which lithium salts are dissolved as electrolyte. Thus, gases which are generated by the Li-ion cell due to side reaction of overcharging are CO, CO2 or other organic gases for which any gas absorbing reaction mechanism has not been established.
Furthermore, the lead-acid cell, the Ni/Cd cell, and the Ni/MH cell have recoverable safety vent which releases oxygen gas which evolves due to side reaction of overcharging and cannot be absorbed in anode active material. On the other hand, the safety vent is not recovered for the Li-ion cell because of preventing from outer moisture. The Li-ion cell thus does not operate once the vent opens. Therefore, as for the Li-ion cell, cell reaction and the safety vent mechanism for keeping safety are largely limited.
In addition, when a plurality of Li-ion cells are placed in series in which degradation of each cell is proceeding, overcharging or over discharging may occur due to imbalance between characteristics of cells so that safety can not be secured. Especially, since the Li-ion cell is expensive, the number of times of exchanging a cell is desired to be as small as possible so as to use the cell over a long time period as possible. However, since safety of the Li-ion cell which is in the last period of its life degrades very much, it is favorable that the cell is exchanged before it is in the unsafe condition. Therefore, for the Li-ion cell, there is an economical problem.
To exchange a cell timely by keeping track of degradation of the cell accurately is one of methods for attaining reliability. As for high energy density cells such as the Ni/MH cell or the Li-ion cell, Smart Battery System (SBS) which was proposed in 1994 is becoming widespread while being modified as a battery management system including charging control and residual capacity evaluation (www.sbs-forum.org can be referred to). However, for controlling and managing cells, only a method based on managing enormous volumes of data is adopted, in which the enormous volumes of data include manufacturing makers, kinds of cells, in addition, data obtained by always monitoring currents, voltages, temperature of cells and the like. Thus, this method is very costly so that prices of products become high.
In addition, monitoring degradation of cells which is important for keeping safety is not regarded as important while keeping and monitoring use time per charge or residual capacity are regarded as important. One of the reason of this is that model changes are often performed for devices which mount the Li-ion cells.
Especially, SBS has only functions of charge control, residual capacity management and the like so that it does not have a function of grasping degradation status of cells. Therefore, exchanging of cells or batteries is performed through user""s intuition.
In addition to SBS, a method of controlling and managing the Li-ion cell used in a video camera is proposed. In this method, since degradation of the cell is judged only by applying previous capacity to relationship between capacity and the number of repetition of charge and discharge which is stored in a memory beforehand, there is a problem in accuracy.
An object of the present invention is to provide a method for performing capacity estimation of the Li-ion cell accurately, a method for performing degradation estimation of the Li-ion cell accurately, an apparatus for performing degradation estimation of the Li-ion cell accurately, and a Li-ion battery which includes a capability for performing degradation estimation accurately.
According to a first aspect of the present invention, the above object of the present invention is achieved by a capacity estimation method for a Li-ion cell, comprising the steps of:
obtaining, when the Li-ion cell is charged by a constant current and constant voltage charging method, an elapsed time from the instant when charge voltage in constant current charge reaches a predetermined voltage to the instant when charge condition is changed from a constant current mode to a constant voltage mode; and
calculating an estimated capacity of the Li-ion cell by using the elapsed time.
According to a second aspect of the present invention, the above object of the present invention is achieved by a capacity estimation method for a Li-ion cell, comprising the steps of:
obtaining, when the Li-ion cell is charged by a constant current and constant voltage charging method, a charge current after a lapse of a predetermined time from the instant when charge condition is changed from a constant current mode to a constant voltage mode; and
calculating an estimated capacity of the Li-ion cell by using the charge current.
According to a third aspect of the present invention, the above object of the present invention is achieved by a capacity estimation method for a Li-ion cell, comprising the steps of:
obtaining, when the Li-ion cell is charged by a constant current and constant voltage charging method, an elapsed time from the instant when charge condition is changed from a constant current mode to a constant voltage mode to the instant when charge current becomes a (0 less than xcex1 less than 1) times of charge current in the constant current mode; and
calculating an estimated capacity of the Li-ion cell by using the elapsed time.